In controlling the liquid level in sumps and tanks there has been a continuing need in the field to be able to turn on or off a pump in response to a liquid level in a tank for the purpose of draining a tank when full, or conversely filling a tank when empty. More specifically there is a need to turn on a pump when a given liquid level is reached, and keep a pump running until a second liquid level is reached.
Current prior art methods of turning on/off or controlling a pump in response to the level of water in a tank include:                1—Single float activating separate high and low level mechanical switches        2—Double floats activating double reed switches        3—Tethered float containing a single tilt sensitive switch whose on/off contact state is a result of the angular orientation the float/switch takes at both water level extremes as a result of being tethered at one end.        4—Tethered float containing double tilt sensitive switches with float operation similar to 3 above.        
The first two methods described above use various forms of switches triggered by a float, which is responsive to the water level, to send a signal to start a pump. In order to maintain the pump running, once the trigger signal is lost, when the float no longer activates the switch, requires some form of latching methodology. The first two methods require and employ an additional holding circuit or relay to perform this latching function and keep the pump running after loss of the trigger signal. This system because of its relative complexity, is relatively expensive and has multiple components which must be maintained.
The third method of level sensing, which is responsive to the angular position of a tethered float switch which, as the water level decreases, passes through a positive up-angle position to a negative down-angle position in relation to the tether pivot point due to being restrained by the tether cable or arm. The up- and down-angle positions determine whether an enclosed tilt sensitive switch is making or breaking the flow of electricity within the switch to the pump. This method does maintain a closed contact for some portion of the total tether travel when the float dwells in the tethered up-angle or down-angled position.
The fourth method is similar to tethered float operation as described above, and similar in electrical operation as 1 and 2 above, using double switch triggering and requiring some additional form of latching circuitry.
It can be seen that the current prior art methods described above require relatively complex systems which are costly and require diligent maintenance to keep operational.